Propylene glycol monoalkyl ethers are high-performance industrial solvents for paints and coatings, cleaners, inks, and a variety of other applications, including agricultural, cosmetic, electronic, textile, and adhesive products. They are also used as chemical intermediates for end-products such as propylene glycol ether acetates.
Typically, propylene glycol monoalkyl ethers are formed by the reaction of propylene oxide with an alcohol, such as methanol or 1-butanol. Although a catalyst is not required, the reaction is typically performed in the presence of a catalyst. A wide variety of catalysts and reaction conditions are taught in the prior art.
The catalysts used in this process include acidic, basic, and neutral species. Particularly useful catalysts include acids such as sulfuric acid, boric acid and some fluorine-containing acids; or bases such as alkali and alkaline earth metal hydroxides and alkoxides, tertiary amines, and certain metal oxides. G.B. Pat. No. 271,169, for instance, discloses the use of sulfuric acid, alkali metal alkoxides, and alkali metal salts of lower fatty acids. U.S. Pat. No. 2,327,053 teaches the use of metal halides such as stannic halides, antimony pentahalides, aluminum halides, zinc halides and ferric halides.
A problem associated with these reactions, and in particular the use of alkali or alkaline earth metal alkoxide catalysts, is that the propylene glycol monoalkyl ether product is contaminated with various carbonyl impurities (such as formaldehyde, acetaldehyde, propionaldehyde, acetone, methoxy acetone, and methoxy butenone) that lead to high UV absorption. In the production of propylene glycol monoalkyl ethers, the product is typically purified by distillation methods. However, distillation fails to remove a significant portion of the carbonyl impurities. For particular applications, it may be necessary to limit the amount of carbonyl impurities and thus lower the UV absorbance of the propylene glycol monoalkyl ether product.
In sum, new methods for the purification of propylene glycol monoalkyl ethers are needed. Particularly useful methods will decrease the amount of carbonyl impurities and thus improve the UV absorbance and color of the propylene glycol monoalkyl ether product. We have discovered an effective, convenient method to purify propylene glycol monoalkyl ethers.